The present invention relates to a fixed-point crucible necessary to calibrate a thermometer, for example, a radiation thermometer, a thermocouple and the like used in a high temperature region exceeding 1000xc2x0 C., a fixed-point temperature realizing apparatus and a thermometer calibrating method using the apparatus.
When thermometers are calibrated, a freezing point or a melting point of metal is employed as a defining fixed-point in the temperature region above room temperature. A fixed-point crucible is used as a method of realizing the fixed-point temperature.
Graphite crucibles, in which pure metal is cast as a fixed-point material of high melting point, have been usually used as the fixed-point crucible. A thermometer is calibrated making use of such a phenomenon that when the liquid phase and the solid phase of the fixed-point material coexist in the crucible, the temperature of the crucible is not varied due to latent heat of melting. This phenomenon is realized by placing the fixed-point crucible in a temperature-variable furnace and observing the temperature variation of the crucible when an environmental temperature is increased and decreased (refer to Chapter 7 of xe2x80x9cNew Edition of Proper Use of Thermometerxe2x80x9d edited by Japan Electric Instrument Manufactures"" Association and published by Nihon Kogyo Publishing Co. (1997)).
The maximum temperature of the fixed-point temperatures is the copper point of 1085xc2x0 C. (freezing point of copper), and the temperature scale is defined by extrapolation in the temperature region higher than the copper point.
The temperature scale in the temperature region above the copper point is maintained making use of a radiation thermometer calibrated at one of the fixed points below the copper point or by transferring the radiance temperature to the current of strip lamp which uses a tungsten ribbon as a filament.
To realize the fixed-point temperature in the temperature region above the copper point, there has been attempted to realize the freezing point of palladium (freezing point; 1550xc2x0 C.) and platinum (freezing point; 1770xc2x0 C.), and an example where the fixed-point was measured by melting them using an alumina crucible has been reported (refer to Quinn, T. J., Chandler T. R. D.: Temperature, Its Measurement and Control in Science and Industry, Plumb, H. H. (ed.), Vol. 4, part 1, p. 295, Pittsburgh: Instrument Society of America (1972), Coates, P. B., Chandler, T. R. D., Andrews, J. W., High Temperature and High Pressure, Vol. 15, p. 573 (1983)).
Further, there has also been reported a trial for obtaining a fixed-point temperature by using tungsten as a crucible material, melting alumina therein, observing the melting and freezing thereof at 2050xc2x0 with a radiation thermometer, and using it as a fixed-point(refer to Sakate, H., Sakuna, F. Ono, A. Metrologia, Vol. 32, p 129 (1995)).
Thermocouples have been calibrated by a palladium wire method, in addition to the calibration carried out at the copper point (1085xc2x0 C.) or the gold point (1064xc2x0 C.). This is a method of inserting a pure palladium wire as pure metal to the distal end of a thermocouple, increasing the temperature of the wire in a heating furnace and observing melting plateaus when the wire is melted.
On the other hand, a trial for using eutectics as a fixed-point temperature has been carried out in metal-metal eutectics. There has been reported that a fixed-point temperature was realized by casting copper-silver eutectics or copper-aluminum eutectics in a graphite crucible and observing the melting and freezing thereof (Itoh, Papers of The Society of Instrumentation and Control Engineers, Vol. 19, No. 12, p. 978 (1983)).
The maximum temperature of conventional fixed-point crucibles which use graphite and to which pure metal is cast is the copper point of 1085xc2x0 C. This is because that when a metal having a higher melting point is melted in a graphite crucible, the graphite is dissolved into the metal and the purity thereof is lowered so that the freezing point is dropped thereby.
Since the temperature scale maintained by the strip lamp or the radiation thermometer, which are employed because a fixed-point crucible above the copper point is not available, depends on the extrapolation, the accuracy of the temperature scale is greatly deteriorated. For example, in the traceability of radiation thermometers in Japan, the provided accuracy at 1085xc2x0 C. is 0.3xc2x0 C., whereas it is abruptly deteriorated to 4xc2x0 C. at 1600xc2x0 C. and to 8xc2x0 C. at 2000xc2x0 C.
Further, it requires enormous efforts to realize temperature scale in high accuracy such as the use of a radiation thermometer of high accuracy, the accurate evaluation of the characteristics of the radiation thermometer. However, since it still depends on the extrapolation, resulting uncertainty in temperature scale realization and maintenance is about 1.2xc2x0 C. at 2000xc2x0 C.
Further, since the emissivity of a strip is not 1 in the strip lamp, correction is needed depending upon a wavelength to be measured. Thus, not only it is difficult to use the strip lamp with high accuracy but also the strip lamp must be used with an inert gas enclosed therein because the tungsten of the ribbon is evaporated at a temperature above 2000xc2x0 C. As a result, stable characteristics of the strip lamp cannot be obtained due to the convection of the inert gas.
For realizing the palladium point or the platinum point using the alumina crucible can be used in the measurement for a short period. However, it lacks practicality because a problem arises in that when the alumina crucible is repeatedly used, the crucible is broken. This is because the alumina is susceptible to thermal shock and made brittle thereby.
Sufficient accuracy cannot be obtained in the calibration of the thermocouple carried out by the palladium wire method. This is because reproducibility obtained thereby is only about 1xc2x0 C. and that accuracy in interpolation is deteriorated because even the copper point as the nearest fixed-point is about 500xc2x0 C. apart
The method of melting alumina in the tungsten crucible is scarcely realized. This is because that the processability of tungsten is bad, it is difficult to enclose molten alumina into a lateral crucible and sufficient accuracy cannot be obtained since a blackbody cavity whose emissivity is near to 1 cannot be formed due to the low emissivity of the tungsten.
The method of using metal/metal eutectics serves the purpose of increasing the number of fixed-point temperatures in a temperature region lower than the copper point. Thus, when the same method is used in a temperature region higher than the copper point, a fixed-point temperature cannot be realized because a freezing point cannot help being lowered by the dissolving of graphite.
An objective of the present invention, which was made to overcome the problems of the conventional art and to realize fixed-point temperatures in a temperature region exceeding the copper point, is to enhance calibrating accuracy in the calibration of all the thermometers which are used in a high temperature region such as a radiation thermometer, a thermocouple and the like.
To achieve the above objective, a fixed-point crucible of the present invention comprises a crucible composed of carbon and having a blackbody cavity or a thermometer well formed therein, and a fixed-point material of high melting point enclosed in the wall of the crucible, wherein the fixed-point material is an eutectic structure of carbon and metal.
Any of iron, cobalt, nickel, palladium, rhodium, platinum, ruthenium, iridium, rhenium, and osmium may be suitably used as the metal used for the fixed-point crucible of the present invention.
A fixed-point temperature realizing apparatus of the present invention may dispose the fixed-point crucible in a temperature-variable furnace, and the temperature-variable furnace may include a heating device for increasing and decreasing the environmental temperature of the crucible and a temperature measuring means capable of measuring the temperature variation of the cavity in the crucible through a thermometer to be calibrated.
More specifically, the fixed-point temperature realizing apparatus of the present invention may use an electric furnace as the temperature-variable furnace, the fixed-point crucible may be accommodated in the electric furnace, the electric furnace may include a heating device composed of a tubular heater which is heated by electrical current flow, a heat insulator for covering the heater and an air tight casing for covering them, and the casing may include a vacuum suction means for evacuating the interior of the furnace to vacuum and an inert gas supply means.
As the tubular heater, a graphite furnace core tube, which is heated by direct current flow, may be used, or a heater element which surrounds an alumina furnace core tube, in which the fixed-point crucible is loaded, may be used.
As a temperature measuring means, a quartz glass sight hole for calibrating a radiation thermometer may be disposed at a position where the blackbody cavity of the crucible can be observed, or a thermometer inserted in alumina protective tube, which reaches the interior of the thermometer well of the crucible from the outside of the casing, may be disposed.
The temperature-variable furnace may be provided with a monitor thermometer for controlling heater power.
A thermometer calibrating method of the present invention using the fixed-point temperature measuring apparatus comprises the steps of disposing in a furnace a fixed-point crucible composed of carbon and a fixed-point material of high melting point enclosed in the wall of the crucible, wherein the fixed-point material is a eutectic structure of carbon and metal, melting and freezing the fixed-point material of high melting point by heating it in the furnace, measuring the temperature variation of the fixed-point material with a thermometer to be calibrated, and calibrating the thermometer based on observed plateaus.
A radiation thermometer, a thermocouple and a resistance thermometer may be particularly suitable as the thermometers to be calibrated by the fixed-point temperature measuring apparatus of the present invention. However, the thermometer to be calibrated is not limited thereto.
Since the above fixed-point temperature measuring crucible of the present invention uses a eutectic structure of carbon and metal as a fixed-point material, the temperature scale in a high temperature region, whose sufficient accuracy cannot be conventionally obtained because it depends on extrapolation from 1085xc2x0 C. of the copper point, can be accurately realized.
According to the fixed-point temperature realizing apparatus and the temperature calibrating method using the apparatus of the present invention, since fixed-point temperatures in the high temperature region can be realized and thermometers such as the radiation thermometer, the thermocouple, the resistance thermometer and the like can be calibrated by interpolation, accuracy can be greatly improved.
The temperature scale can be maintained by only using fixed-points and a radiation thermometer without the use of a standard strip lamp which is conventionally used to maintain the temperature scale.
Further, since calibration can be carried out by interpolation, the accurate evaluation of the characteristics of a highly-accurate standard radiation thermometer, which is conventionally required, is unnecessary, whereby a calibration job is remarkably simplified and a system for providing a temperature scale can be upgraded.
Furthermore, the palladium wire method is unnecessary in the calibration of the thermocouple. In addition, in the development of a high temperature thermocouple which is carried out hereinafter, the characteristics of the thermocouple such as the stability, the variation among different thermocouples, and the like can be evaluated with high accuracy, which can also contribute to the improvement of the characteristics of the thermocouple.